In order to transmit facsimile information over the phone lines in some type of secured manner, encrypting is required at one end and decrypting is required at the opposite end. This results in a secure fax transmission. However, most secure transmissions require the same type of hardware on both ends in order to provide compatibility. Due to the large number of different systems that have been in the marketplace for providing secure facsimile transmission, standards have evolved. These standards have primarily come about as a result of secure facsimile by the military community utilizing a Secure Telephone Unit (STU).
Initially, the military standard 188 (MIL-STD-188) existed to cover tactical and long-haul communication system technical standards. However, this later evolved into MIL-STD-188C. Further evolution resulted in a MIL-STD-188-100, a series that covered common standards for tactical and long-haul communications, a MIL-STD-188-200 series covering standards for tactical communications only, and a MIL-STD-188-300 series covering standards for long-haul applications only. The 100 series standards have been primarily directed toward the tactical and long-haul communications, which resulted in the recent military standards MIL-STD-188-161. This military standard provides parameters that are compatible with the mandatory parameters of standardization agreement (STANG) 5000 and CCITT Group 3 equipment. This military standard was issued on Jul. 4, 1988, which superseded the MIL-STD-188-161 standard issued on Jan. 30, 1981. It is generally entitled "Interoperability and Performance Standards for Digital Facsimile Equipment", which military standard is incorporated herein by reference.
When the initial military standard 161 was issued, a number of facsimile equipment manufacturers put out dedicated secured FAX systems. These secure facsimile systems incorporated hardware that would communicate with the STU which was connected to the phone line. This STU is provided by the government, and outputs on an RS232 port a serial data stream that complies with the military standard. However, one disadvantage to most of the secure facsimile equipment is that it is self-contained; that is, the hardware to communicate with the STU was internal to the facsimile machine, and the facsimile machine merely provided an RS232 port for communication. This resulted in a relatively expensive machine, which generally required that the machine on the opposite end of the communication path be essentially the same machine. In fact, in the early days of the MIL-STD-166, most communications required the same machine to be at either end of the communication path. It was only later in the evolution of the machines that they became somewhat compatible.
With the advent of facsimile machines to the general public, the prices for the general Group 3 machines have been decreasing. Therefore, there has been a desire to utilize the relatively inexpensive facsimile machines and provide some kind of interface with the STU. Early attempts at this problem resulted in an interface that would allow the Group 3 facsimile machine to communicate through the STU, but generally required that the opposite end of the communication path also have an interface that was identical. In general, the interface that allowed a generic Group 3 machine to be utilized with a STU would not communicate with anything other than a generic Group 3 machine on the opposite end of the communication path that utilized a similar or identical interface. Once could not communicate with another type of facsimile machine that did not have the interface.